include/boost/corosio/native/detail/select/select_scheduler.hpp

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select_scheduler.hpp

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1		//
2		// Copyright (c) 2026 Steve Gerbino
3		// Copyright (c) 2026 Michael Vandeberg
4		//
5		// Distributed under the Boost Software License, Version 1.0. (See accompanying
6		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7		//
8		// Official repository: https://github.com/cppalliance/corosio
9		//
10
11		#ifndef BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
12		#define BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
13
14		#include <boost/corosio/detail/platform.hpp>
15
16		#if BOOST_COROSIO_HAS_SELECT
17
18		#include <boost/corosio/detail/config.hpp>
19		#include <boost/capy/ex/execution_context.hpp>
20
21		#include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp>
22		#include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp>
23
24		#include <boost/corosio/native/detail/select/select_traits.hpp>
25		#include <boost/corosio/detail/timer_service.hpp>
26		#include <boost/corosio/native/detail/make_err.hpp>
27		#include <boost/corosio/native/detail/posix/posix_resolver_service.hpp>
28		#include <boost/corosio/native/detail/posix/posix_signal_service.hpp>
29		#include <boost/corosio/native/detail/posix/posix_stream_file_service.hpp>
30		#include <boost/corosio/native/detail/posix/posix_random_access_file_service.hpp>
31
32		#include <boost/corosio/detail/except.hpp>
33
34		#include <sys/select.h>
35		#include <unistd.h>
36		#include <errno.h>
37		#include <fcntl.h>
38
39		#include <atomic>
40		#include <chrono>
41		#include <cstdint>
42		#include <limits>
43		#include <mutex>
44		#include <new>
45		#include <unordered_map>
46
47		namespace boost::corosio::detail {
48
49		struct select_op;
50
51		/** POSIX scheduler using select() for I/O multiplexing.
52
53		This scheduler implements the scheduler interface using the POSIX select()
54		call for I/O event notification. It inherits the shared reactor threading
55		model from reactor_scheduler: signal state machine, inline completion
56		budget, work counting, and the do_one event loop.
57
58		The design mirrors epoll_scheduler for behavioral consistency:
59		- Same single-reactor thread coordination model
60		- Same deferred I/O pattern (reactor marks ready; workers do I/O)
61		- Same timer integration pattern
62
63		Known Limitations:
64		- FD_SETSIZE (~1024) limits maximum concurrent connections
65		- O(n) scanning: rebuilds fd_sets each iteration
66		- Level-triggered only (no edge-triggered mode)
67
68		@par Thread Safety
69		All public member functions are thread-safe.
70		*/
71		class BOOST_COROSIO_DECL select_scheduler final : public reactor_scheduler
72		{
73		public:
74		/** Construct the scheduler.
75
76		Creates a self-pipe for reactor interruption.
77
78		@param ctx Reference to the owning execution_context.
79		@param concurrency_hint Hint for expected thread count (unused).
80		*/
81		select_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
82
83		/// Destroy the scheduler.
84		~select_scheduler() override;
85
86		select_scheduler(select_scheduler const&) = delete;
87		select_scheduler& operator=(select_scheduler const&) = delete;
88
89		/// Shut down the scheduler, draining pending operations.
90		void shutdown() override;
91
92		/** Return the maximum file descriptor value supported.
93
94		Returns FD_SETSIZE - 1, the maximum fd value that can be
95		monitored by select(). Operations with fd >= FD_SETSIZE
96		will fail with EINVAL.
97
98		@return The maximum supported file descriptor value.
99		*/
100		static constexpr int max_fd() noexcept
101		{
102		return FD_SETSIZE - 1;
103		}
104
105		/** Register a descriptor for persistent monitoring.
106
107		The fd is added to the registered_descs_ map and will be
108		included in subsequent select() calls. The reactor is
109		interrupted so a blocked select() rebuilds its fd_sets.
110
111		@param fd The file descriptor to register.
112		@param desc Pointer to descriptor state for this fd.
113
114		@return The error if the fd cannot be tracked, otherwise a
115		default constructed error code.
116		*/
117		std::error_code
118		register_descriptor(int fd, reactor_descriptor_state* desc) const;
119
120		/** Deregister a persistently registered descriptor.
121
122		@param fd The file descriptor to deregister.
123		*/
124		void deregister_descriptor(int fd) const;
125
126		/** Interrupt the reactor so it rebuilds its fd_sets.
127
128		Called when a write, connect, or write-wait op is registered
129		after the reactor's snapshot was taken. Without this,
130		select() may block not watching for writability on the fd.
131		*/
132		void notify_reactor() const;
133
134		/// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp).
135		[[nodiscard]] std::error_code
136	41x	register_signal_reader(int read_fd) override
137		{
138	41x	return register_descriptor(read_fd, signal_pipe_reader_.arm());
139		}
140
141		private:
142		void
143		run_task(lock_type& lock, context_type* ctx,
144		long timeout_us) override;
145		void interrupt_reactor() const override;
146		long calculate_timeout(long requested_timeout_us) const;
147
148		// Watches the global signal self-pipe's read end (armed lazily by
149		// register_signal_reader on the first signal registration).
150		reactor_signal_pipe_reader signal_pipe_reader_;
151
152		// Self-pipe for interrupting select()
153		int pipe_fds_[2]; // [0]=read, [1]=write
154
155		// Per-fd tracking for fd_set building
156		mutable std::unordered_map<int, reactor_descriptor_state*> registered_descs_;
157		mutable int max_fd_ = -1;
158		};
159
160	701x	inline select_scheduler::select_scheduler(capy::execution_context& ctx, int)
161	701x	: pipe_fds_{-1, -1}
162	701x	, max_fd_(-1)
163		{
164	701x	if (::pipe(pipe_fds_) < 0)
165	✗	detail::throw_system_error(make_err(errno), "pipe");
166
167	2103x	for (int i = 0; i < 2; ++i)
168		{
169	1402x	int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
170	1402x	if (flags == -1)
171		{
172	✗	int errn = errno;
173	✗	::close(pipe_fds_[0]);
174	✗	::close(pipe_fds_[1]);
175	✗	detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
176		}
177	1402x	if (::fcntl(pipe_fds_[i], F_SETFL, flags \| O_NONBLOCK) == -1)
178		{
179	✗	int errn = errno;
180	✗	::close(pipe_fds_[0]);
181	✗	::close(pipe_fds_[1]);
182	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
183		}
184	1402x	if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
185		{
186	✗	int errn = errno;
187	✗	::close(pipe_fds_[0]);
188	✗	::close(pipe_fds_[1]);
189	✗	detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
190		}
191		}
192
193	701x	timer_svc_ = &get_timer_service(ctx, *this);
194	701x	timer_svc_->set_on_earliest_changed(
195	4882x	timer_service::callback(this, [](void* p) {
196	4181x	static_cast<select_scheduler*>(p)->interrupt_reactor();
197	4181x	}));
198
199	701x	get_resolver_service(ctx, *this);
200	701x	get_signal_service(ctx, *this);
201	701x	get_stream_file_service(ctx, *this);
202	701x	get_random_access_file_service(ctx, *this);
203
204	701x	completed_ops_.push(&task_op_);
205	701x	}
206
207	1402x	inline select_scheduler::~select_scheduler()
208		{
209	701x	if (pipe_fds_[0] >= 0)
210	701x	::close(pipe_fds_[0]);
211	701x	if (pipe_fds_[1] >= 0)
212	701x	::close(pipe_fds_[1]);
213	1402x	}
214
215		inline void
216	701x	select_scheduler::shutdown()
217		{
218	701x	shutdown_drain();
219
220	701x	if (pipe_fds_[1] >= 0)
221	701x	interrupt_reactor();
222	701x	}
223
224		inline std::error_code
225	7118x	select_scheduler::register_descriptor(
226		int fd, reactor_descriptor_state* desc) const
227		{
228	7118x	if (fd < 0 \|\| fd >= FD_SETSIZE)
229	✗	return make_err(EMFILE);
230
231	7118x	desc->registered_events = reactor_event_read \| reactor_event_write;
232	7118x	desc->fd = fd;
233	7118x	desc->scheduler_ = this;
234	7118x	desc->mutex.set_enabled(reactor_io_locking_);
235	7118x	desc->ready_events_.store(0, std::memory_order_relaxed);
236
237		{
238	7118x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
239	7118x	desc->impl_ref_.reset();
240	7118x	desc->read_ready = false;
241	7118x	desc->write_ready = false;
242	7118x	}
243
244		{
245	7118x	mutex_type::scoped_lock lock(mutex_);
246		try
247		{
248	7118x	registered_descs_[fd] = desc;
249		}
250	✗	catch (std::bad_alloc const&)
251		{
252	✗	return make_err(ENOMEM);
253	✗	}
254	7118x	if (fd > max_fd_)
255	7108x	max_fd_ = fd;
256	7118x	}
257
258	7118x	interrupt_reactor();
259	7118x	return {};
260		}
261
262		inline void
263	7077x	select_scheduler::deregister_descriptor(int fd) const
264		{
265	7077x	mutex_type::scoped_lock lock(mutex_);
266
267	7077x	auto it = registered_descs_.find(fd);
268	7077x	if (it == registered_descs_.end())
269	✗	return;
270
271	7077x	registered_descs_.erase(it);
272
273	7077x	if (fd == max_fd_)
274		{
275	6921x	max_fd_ = pipe_fds_[0];
276	13533x	for (auto& [registered_fd, state] : registered_descs_)
277		{
278	6612x	if (registered_fd > max_fd_)
279	6557x	max_fd_ = registered_fd;
280		}
281		}
282	7077x	}
283
284		inline void
285	3392x	select_scheduler::notify_reactor() const
286		{
287	3392x	interrupt_reactor();
288	3392x	}
289
290		inline void
291	15971x	select_scheduler::interrupt_reactor() const
292		{
293	15971x	char byte = 1;
294	15971x	[[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
295	15971x	}
296
297		inline long
298	432368x	select_scheduler::calculate_timeout(long requested_timeout_us) const
299		{
300	432368x	if (requested_timeout_us == 0)
301	✗	return 0;
302
303	432368x	auto nearest = timer_svc_->nearest_expiry();
304	432368x	if (nearest == timer_service::time_point::max())
305	607x	return requested_timeout_us;
306
307	431761x	auto now = std::chrono::steady_clock::now();
308	431761x	if (nearest <= now)
309	637x	return 0;
310
311		auto timer_timeout_us =
312	431124x	std::chrono::duration_cast<std::chrono::microseconds>(nearest - now)
313	431124x	.count();
314
315	431124x	constexpr auto long_max =
316		static_cast<long long>((std::numeric_limits<long>::max)());
317		auto capped_timer_us =
318	431124x	(std::min)((std::max)(static_cast<long long>(timer_timeout_us),
319	431124x	static_cast<long long>(0)),
320	431124x	long_max);
321
322	431124x	if (requested_timeout_us < 0)
323	431124x	return static_cast<long>(capped_timer_us);
324
325		return static_cast<long>(
326	✗	(std::min)(static_cast<long long>(requested_timeout_us),
327	✗	capped_timer_us));
328		}
329
330		inline void
331	469285x	select_scheduler::run_task(
332		lock_type& lock, context_type* ctx, long timeout_us)
333		{
334		long effective_timeout_us =
335	469285x	task_interrupted_ ? 0 : calculate_timeout(timeout_us);
336
337		// Snapshot registered descriptors while holding lock.
338		// Record which fds need write monitoring to avoid a hot loop:
339		// select is level-triggered so writable sockets (nearly always
340		// writable) would cause select() to return immediately every
341		// iteration if unconditionally added to write_fds. Membership
342		// stays opt-in: a parked write wait opts in the same way a
343		// parked write or connect op does.
344		struct fd_entry
345		{
346		int fd;
347		reactor_descriptor_state* desc;
348		bool needs_write;
349		};
350		fd_entry snapshot[FD_SETSIZE];
351	469285x	int snapshot_count = 0;
352
353	1178656x	for (auto& [fd, desc] : registered_descs_)
354		{
355	709371x	if (snapshot_count < FD_SETSIZE)
356		{
357	709371x	conditionally_enabled_mutex::scoped_lock desc_lock(desc->mutex);
358	709371x	snapshot[snapshot_count].fd = fd;
359	709371x	snapshot[snapshot_count].desc = desc;
360	709371x	snapshot[snapshot_count].needs_write =
361	1404630x	(desc->write_op \|\| desc->connect_op \|\|
362	695259x	desc->wait_write_op);
363	709371x	++snapshot_count;
364	709371x	}
365		}
366
367	469285x	if (lock.owns_lock())
368	432369x	lock.unlock();
369
370	469285x	task_cleanup on_exit{this, &lock, ctx};
371
372		fd_set read_fds, write_fds, except_fds;
373	7977845x	FD_ZERO(&read_fds);
374	7977845x	FD_ZERO(&write_fds);
375	7977845x	FD_ZERO(&except_fds);
376
377	469285x	FD_SET(pipe_fds_[0], &read_fds);
378	469285x	int nfds = pipe_fds_[0];
379
380	1178656x	for (int i = 0; i < snapshot_count; ++i)
381		{
382	709371x	int fd = snapshot[i].fd;
383	709371x	FD_SET(fd, &read_fds);
384	709371x	if (snapshot[i].needs_write)
385	14116x	FD_SET(fd, &write_fds);
386	709371x	FD_SET(fd, &except_fds);
387	709371x	if (fd > nfds)
388	468863x	nfds = fd;
389		}
390
391		struct timeval tv;
392	469285x	struct timeval* tv_ptr = nullptr;
393	469285x	if (effective_timeout_us >= 0)
394		{
395	468681x	tv.tv_sec = effective_timeout_us / 1000000;
396	468681x	tv.tv_usec = effective_timeout_us % 1000000;
397	468681x	tv_ptr = &tv;
398		}
399
400	469285x	int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
401
402		// EINTR: signal interrupted select(), just retry.
403		// EBADF: an fd was closed between snapshot and select(); retry
404		// with a fresh snapshot from registered_descs_.
405	469285x	if (ready < 0)
406		{
407	✗	if (errno == EINTR \|\| errno == EBADF)
408	✗	return;
409	✗	detail::throw_system_error(make_err(errno), "select");
410		}
411
412		// Process timers outside the lock
413	469285x	timer_svc_->process_expired();
414
415	469285x	ready_queue local_ops;
416
417	469285x	if (ready > 0)
418		{
419	443837x	if (FD_ISSET(pipe_fds_[0], &read_fds))
420		{
421		char buf[256];
422	14726x	while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0)
423		{
424		}
425		}
426
427	1078914x	for (int i = 0; i < snapshot_count; ++i)
428		{
429	635077x	int fd = snapshot[i].fd;
430	635077x	reactor_descriptor_state* desc = snapshot[i].desc;
431
432	635077x	std::uint32_t flags = 0;
433	635077x	if (FD_ISSET(fd, &read_fds))
434	440061x	flags \|= reactor_event_read;
435	635077x	if (FD_ISSET(fd, &write_fds))
436	3387x	flags \|= reactor_event_write;
437	635077x	if (FD_ISSET(fd, &except_fds))
438	✗	flags \|= reactor_event_error;
439
440	635077x	if (flags == 0)
441	191640x	continue;
442
443	443437x	desc->add_ready_events(flags);
444
445	443437x	bool expected = false;
446	443437x	if (desc->is_enqueued_.compare_exchange_strong(
447		expected, true, std::memory_order_release,
448		std::memory_order_relaxed))
449		{
450	443437x	local_ops.push(desc);
451		}
452		}
453		}
454
455	469285x	lock.lock();
456
457	469285x	completed_ops_.splice(local_ops);
458	469285x	}
459
460		} // namespace boost::corosio::detail
461
462		#endif // BOOST_COROSIO_HAS_SELECT
463
464		#endif // BOOST_COROSIO_NATIVE_DETAIL_SELECT_SELECT_SCHEDULER_HPP
465